Income Taxes - Components of Federal Corporate Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Current income tax expense	$ 30,634	$ 28,316	$ 25,317
Deferred income tax expense	(1,849)	789	2,625
Income tax expense	$ 28,785	$ 29,105	$ 27,942